http://blog.domaintools.com/2007/09/domain-renewal-accounting-loophole-exposed-in-verisign-registry/
Some retail-registrars may be taking their registrants for a ride, forcing renewal payments and then not refunding those payments after outbound transfer (although the central-registry provides a credit afterward).
Quote: "The special circumstances are as follows: The domain is past expiration, the owner renews the domain at the current registrar, the owner then transfers away with in 45 days of the anniversary of the creation date… Do not renew you domain at your old registrar during the grace period and then transfer out. Instead, directly transfer out or pay your renewal fee, wait until day 46 after the old expiration and then transfer out. You will loose one year of registration if you pay first then leave."
I can smell the class-action coming. 
Another alternative is to pay your renewals far in advance to avoid ever having to deal with the issues surrounding this sort of thing.. I always top up my gas tank in advance.. Never wait till you’re below a quarter tank before filling up and you’ll never be walking to the gas station with a 
jerry-can.
Terrific observation on Jay Westerdal’s blog though… his posts just get better and better.